A group of nearly 400 lawmakers on both sides of the aisle is calling on the Trump administration to extend a United Nations arms embargo on Iran, which expires in October.

In a new letter to Secretary of State Mike Pompeo, 387 bipartisan members of the House of Representatives on Monday urged the State Department to practice “robust diplomacy” in an effort to renew the embargo as well as travel restrictions on people aiding in Iran’s proliferation activities.

“We are concerned that the ban’s expiration will lead to more states buying and selling weapons to and from Iran,” the members wrote. “Additionally, states concerned about Iran’s malign activities may feel they do not have sufficient legal authority to stop transfers once the U.N. embargo expires.”

They urged the department to work with “allies and like-minded partners” to rally support to extend the embargo and “make clear to the international community that U.S. sanctions on Iranian arms transfers remain in place and will be fully enforced.”

The group was led by House Foreign Affairs Committee Chairman Eliot Engel, New York Democrat, and ranking member Michael McCaul, Texas Republican, as well as Reps. Stephanie Murphy, Florida Democrat, and Brian Fitzpatrick, Pennsylvania Republican, and encompasses more than three-quarters of all House members.

The current version of the arms embargo and arms export ban on Iran was adopted in 2015 in conjunction with a key nuclear accord between Iran and several world powers, including the U.S., and is set to lift on Oct. 18.

President Trump withdrew from the Iran nuclear deal in 2018 despite European opposition, a move that allowed the U.S. to reinstate economic sanctions against Tehran that had been waived under the deal and add new sanctions.

The administration has since threatened to launch a resurgence of all U.N. sanctions in a move to gain backing from the 15-member security council to extend the arms embargo.

But the possibility of a renewal has been met with severe opposition from China and Russia — both permanent members of the security council with veto power.

The group cited Iran’s repeated violations of the resolution with arms exports to Syria, Iraq and Yemen over the past several years.

“Iran’s illicit transfers of weapons directly contribute to some of the most destabilizing threats to the United States and our partners in the Middle East such as Israel and the Gulf States,” they wrote. “Even so, the resolution serves as an important means to restrain the sale of weapons to Iran and makes clear continued international resolve to stop Iran’s illicit weapons transfers.”

Earlier Monday, Iran slammed U.S. efforts to continue the embargo on the country, calling the move “illegitimate.”

“Iran is not seeking to exit the 2015 nuclear deal with six powers,” Abbas Mousavi, a spokesman for Iran’s Ministry of Foreign Affairs, said in a televised news briefing. “America’s move is illegitimate and our reaction will be proportionate.”

Last week, U.S. special envoy to Iran Brian Hook told reporters he is “hopeful” efforts to extend the restrictions will be successful.

He said the administration’s focus “is on engaging in thoughtful and measured diplomacy with all the relevant parties to successfully negotiate a renewal of the U.N. arms embargo,” adding that he is not seeking to extend the embargo quickly.

Mr. Pompeo has vowed to not allow Iran to have the ability to purchase conventional weapons currently prohibited under the U.N. embargo.

“We’ll work with the U.N. Security Council to extend that prohibition on those arms sales,” he told reporters last week. “And then in the event we can’t get anyone else to act, the United States is evaluating every possibility about how we might do that.”

The latest dispute comes amid heightened tensions between the U.S. and Iran after Mr. Trump said he has authorized the Navy to “shoot down and destroy any and all Iranian gunboats if they harass” American ships.

His comments came a week after a handful of Iran’s Islamic Revolutionary Guard Corps boats made threatening approaches to U.S. warships conducting joint operations with Army attack helicopters in international waters.

“America must continue its longstanding, bipartisan leadership in order to limit Iran’s destabilizing activities throughout the world,” the lawmakers wrote. “We look forward to working with you to re-authorize these expiring U.N. restrictions, which are essential to protecting our national security and the American people.”